I created a small Java client for Alfresco using Apache Chemistry. Using the code below I see that Alfresco returns appropriate types.

Sample code:

import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;

import org.apache.chemistry.opencmis.client.bindings.CmisBindingFactory;
import org.apache.chemistry.opencmis.commons.SessionParameter;
import org.apache.chemistry.opencmis.commons.data.Properties;
import org.apache.chemistry.opencmis.commons.data.PropertyData;
import org.apache.chemistry.opencmis.commons.spi.CmisBinding;

public class App {
	public static void main(String[] args) {
		Map<String, String> parameters = new HashMap<String, String>();
		parameters.put(SessionParameter.ATOMPUB_URL,
				"http://localhost:8080/alfresco/api/-default-/public/cmis/versions/1.0/atom");
		parameters.put(SessionParameter.USER, "user");
		parameters.put(SessionParameter.PASSWORD, "password");

		CmisBinding binding = CmisBindingFactory.newInstance().createCmisAtomPubBinding(parameters);

		String repositoryId = "-default-";
		String objectId = "dc9040ad-0a92-4e64-afa5-5e70eff1f597";
		Properties properties = binding.getObjectService().getProperties(repositoryId, objectId, null, null);
		
		List<PropertyData<?>> propertiesList = properties.getPropertyList();
		Iterator<PropertyData<?>> it = propertiesList.iterator();
		while (it.hasNext()) {
			PropertyData<?> values = it.next();
			System.out.println(values);

			Object firstValue = values.getFirstValue();
			System.out.println(null != firstValue ? firstValue.getClass().getName() : "");
		}
	}
}

Prints something like:

Property [id=alfcmis:nodeRef, display Name=Alfresco Node Ref, local name=nodeRef, query name=alfcmis:nodeRef, values=[workspace://SpacesStore/dc9040ad-0a92-4e64-afa5-5e70eff1f597]][extensions=null]
java.lang.String
Property [id=cmis:isImmutable, display Name=Is Immutable, local name=isImmutable, query name=cmis:isImmutable, values=[false]][extensions=null]
java.lang.Boolean
Property [id=cmis:creationDate, display Name=Creation Date, local name=creationDate, query name=cmis:creationDate, values=[java.util.GregorianCalendar[time=?,areFieldsSet=false,areAllFieldsSet=false,lenient=true,zone=sun.util.calendar.ZoneInfo[id="GMT",offset=0,dstSavings=0,useDaylight=false,transitions=0,lastRule=null],firstDayOfWeek=1,minimalDaysInFirstWeek=1,ERA=?,YEAR=2017,MONTH=7,WEEK_OF_YEAR=?,WEEK_OF_MONTH=?,DAY_OF_MONTH=9,DAY_OF_YEAR=?,DAY_OF_WEEK=?,DAY_OF_WEEK_IN_MONTH=?,AM_PM=?,HOUR=?,HOUR_OF_DAY=8,MINUTE=4,SECOND=30,MILLISECOND=816,ZONE_OFFSET=?,DST_OFFSET=?]]][extensions=null]
java.util.GregorianCalendar
Property [id=cmis:contentStreamLength, display Name=Content Stream Length, local name=contentStreamLength, query name=cmis:contentStreamLength, values=[3369868]][extensions=null]
java.math.BigInteger
...